MRI scanning processes are sensitive to movement of the patient's anatomy that is being scanned. It takes time to acquire a complete k-space data set (the Fourier data set from which an MRI image is computed), typically a minute or so, and any movement will result in different parts of the sampled k-space data set being obtained with the patient's anatomy in different positions. This inconsistency in the data set will create motion artefacts in the final image. The motion of the anatomy may be due to the respiratory and cardiac cycles of the patient.
A similar problem arises during radiotherapy; respiration causes tumours or other lesions in the chest area to move in synchrony. This presents problems in targeting the radiation at the tumour, as the tumour's position at any one time is uncertain. To achieve the primary objective of radiotherapy which is to irradiate the tumour, a margin around the nominal tumour position is used to compensate for this uncertainty, meaning that additional healthy tissue is irradiated.
To limit MRI artefacts during scanning and to reduce the radiotherapy treatment margin, measures are taken to control the patient's breathing during a procedure. These include simply asking the patient to hold their breath, sometimes with the assistance of an audible or visual prompt, or the use of devices such as respiratory belts, skin-mounted markers and the like which offer proxy data relating to respiration. A further alternative is the “Active Breathing Control” device, or “ABC”, which comprises a face mask or breathing tube through which the patient breathes and which includes a pneumotachograph for measuring air flow rate. This rate information is integrated to produce lung filling information, and a valve in the flow path is closed to enforce a breath hold at a specific lung filling volume. The aim is to produce repeated static episodes in which the patient's anatomy is in a reproducible position, which can then be used for radiotherapy or for k-space data acquisition.
In MRI imaging, it is also possible to retrospectively select k-space data on the basis of respiratory-cycle information that was acquired during scanning. Most MRI apparatus allows for a fast acquisition of 1-dimensional line data or 2-dimensional slice data, the former being usually known as the “navigator channel”. This can be used to identify features within the anatomy such as the diaphragm, from which the breathing phase can be determined. By selecting k-space data taken at like points in the breathing phase, an image without respiratory artefacts can be created.
U.S. Pat. No. 7,393,329 (Wong et al) suggests using an ABC device during radiotherapy, gating delivery of radiotherapy to periods of enforced breath hold. Wong et al (incorporated herein by reference) provides a good explanation of the operation of an ABC, and the reader is directed to Wong et al for a fuller understanding of the present invention.
Arnold et al (“Lung MRI Using an MR-Compatible Active Breathing Control (MR-ABC)”), Magnetic Resonance in Medicine 58:1092-1098 (2007) suggest combining an ABC device with an ECG to monitor cardiac activity during MRI scanning. Using an enforced breath hold of 1.5 seconds means that at least one cardiac cycle will take place during the breath hold, allowing k-space acquisition to be triggered by the cardiac R-wave to capture an image of a completely stationary anatomy. It also teaches triggering the valve to close when a flow reversal is detected rather that at a specific lung filling volume. This causes a breath hold at maximum exhalation, which is said to be a more reproducible point. The MR data acquisition is triggered indirectly; the valve trigger is fed to the ECG, which then produces a pulse after the next cardiac R-wave, and the pulse initiates the data acquisition.